I have used a dubbing loop made on the fly with the working silk, the turbo dubbing brush maker, split thread, and Leisenring's method. All have their advantages for different types of flies but for a sparse, transparent, wispy wet fly body it's hard to beat Leisenring's method. I really don't know why it works so well but I think cholcomb13 is probably right about the horizontal vs. vertical thing.
Joe